Judging from the notes, x3 is supposed to be the side of x2 which x1 is on (x1 is to the x3 side of x2), and x4 is supposed to tell you which way x2 is facing. (The definition of mlana definitely lacks clarity.)

le termla be le kubli bei le fagri cu glare"The side of the cube facing the fire is hot."

le sicni cu mlana le kelkubli fo le sinxa be li pa sefta"The coin is to the side of the die if you think of the 1 side as the front of the die."